1. Field of Invention
The invention relates to an automatic transmission provided in vehicles, and more particularly, to a hydraulic control system for controlling engaging elements in the transmission mechanism.
2. Description of Related Art
In the systems in which electrically controlled solenoid valves supply a hydraulic pressure to respective hydraulic servos of a plurality of engaging elements, the hydraulic pressure may be simultaneously supplied from a plurality of solenoid valves in the event of the electrical failure or sticking of the valves (hereinafter, collectively referred to as "failure"). In such a case, three or more of the engaging elements may be simultaneously engaged, whereby the transmission may be interlocked. In order to prevent such a problem, a valve for supplying the hydraulic pressure only to two hydraulic servos is provided in the system. Such conventional art is disclosed, for example, in U.S. Pat. No. 4,903,551 or U.S. Pat. No. 5,010,786.
The conventional art is structured to ensure that, in the case of the failure, the hydraulic pressure is supplied to the hydraulic servos such that two engaging elements attaining the third speed are engaged. The third speed is assured because, at the third speed, a certain driving force required for limphome control in the forward four-speed gear train of the conventional art can be obtained, while preventing an abrupt engine brake resulting from shifting to the lower gear even upon failure at the highest, fourth speed.
In the case where the higher speed (e.g., the third speed in the above example) is ensured in the event of the failure, the vehicle may possibly fail to climb a steep hill during the limphome control, and/or may fail to run over a slight difference in level of the road during running at the low speed. However, if a lower speed is assured upon failure, an abrupt engine brake may occur. In view of the above, it is desirable that the running state be changed before and after the recognition of the failure by the driver, and that the speed to be attained is varied according to the driving force required to change the running state. In spite of such requirements, it is practically difficult to shift the gear during the failure.